A BACKGROUND SODIUM CONDUCTANCE IS NECESSARY FOR SPONTANEOUS DEPOLARIZATIONS IN RAT PITUITARY CELL-LINE GH(3)

The role of Na+ in the expression of membrane potential activity in th e clonal rat pituitary cell line GH(3) was investigated using the perf orated patch variation of patch-clamp electrophysiological techniques. It was found that replacing bath Na+ with choline, tris(hydroxymethyl ) aminomethane (Tris), or N-methyl-D-glucamine (NMG) caused the cells to hyperpolarize 20-30 mV. Tetrodotoxin had no effect. The effects of the Na+ substitutes could not be explained by effects on potassium or calcium currents. Although all three Na+ substitutes suppressed voltag e-dependent calcium current by 10-20%, block of voltage-dependent calc ium current by nifedipine or Co2+ did not result in hyperpolarization of the cells. There was no effect of the Nat substitutes on voltage-de pendent potassium currents. In contrast, all three Na+ substitutes inf luenced calcium-activated potassium currents [I-K(Ca)], but only at de polarized potentials. Choline consistently suppressed I-K(Ca), whereas Tris and NMG either had no effect or slightly increased I-K(Ca) These effects on I-K(Ca) also cannot explain the hyperpolarization induced by removing bath Na+ Choline always hyperpolarized cells yet suppresse d I-K(Ca) Furthermore, removing bath Na+ caused an increase in cell in put resistance, an observation consistent with the loss of a membrane conductance as the basis of the hyperpolarization. Direct measurement of background currents revealed a 12-pA inward current at -84 mV that was lost upon removing bath Na+. These results suggest that this backg round sodium conductance provides the depolarizing drive for GH(3) cel ls to reach the threshold for firing calcium-dependent action potentia ls.